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Electronic Supplementary Information (ESI)

P2-type Na0.7(Ni0.6Co0.2Mn0.2)O2 cathode with excellent cyclability and rate

capability for sodium ion batteries

Jonghyun Choi, Kyeong-Ho Kim, Chul-Ho Jung, and Seong-Hyeon Hong*

Department of Materials Science and Engineering and Research Institute of Advanced

Materials, Seoul National University, Seoul 151-744, Republic of Korea

* Corresponding author

Prof. Seong-Hyeon Hong (S.-H. Hong)

E-mail: shhong@snu.ac.kr

Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2019

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Experimental

Sample preparation

Spherical (Ni0.6Co0.2Mn0.2)(OH)2 precursors were synthesized using co-precipitation method

using NiSO4·6H2O, CoSO4·7H2O and MnSO4·H2O (Daejung Chemical Co.) as starting

materials. A homogeneously mixed transition metal sulfates were slowly pumped into a

continuously stirred tank reactor. Concurrently, 4 M NaOH (aq) and 2 M NH4OH chelating

agent (aq) were also separately pumped into the reactor. After reaction, the precursor powders

were obtained by filtering, washing, and vacuum drying at 110 °C for 12 h. For a typical

synthesis of P2-type Na-MCM, the mixture of Na2CO3 and (Ni0.6Co0.2Mn0.2)(OH)2 with a molar

ratio of 0.7:1 was thoroughly ground, pressed into pellet, and annealed at 400 °C for 5 h and at

780 °C for 15 h in air followed by the quench process. The O3-type Na-NCM was prepared

using Na2CO3 and (Ni0.6Co0.2Mn0.2)(OH)2 with a molar ratio of 1.05:1. The mixture was

annealed 650 °C for 15 h in air followed by furnace cooling process. Finally, the materials were

immediately transferred to a glovebox under inert atmosphere.

Materials Characterization

The chemical composition of the synthesized powders was determined using inductively

coupled plasma atomic emission spectrometer (ICP-AES; OPTIMA 4300DV, Perkin-Elmer).

The crystal structure was determined by synchrotron radiation powder X-ray diffraction

(SPXRD) data collected at room temperature from the 9B HRPD beamline of the Pohang

Accelerator Laboratory (PAL) and X-ray diffraction (XRD, D8 Advance, Bruker). The

Rietveld refinement was performed by Fullprof program. The morphology was observed by

scanning electron microscopy (SEM, SU-70, Hitachi). The Ni K-edge, Co K-edge and Mn K-

edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine

structure (EXAFS) were collected on the BL10C beam line at the Pohang Light Source (PLS-

II) in Korea with top-up mode operation under a ring current of 200 mA at 3.0 GeV.

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Electrochemical measurements

The cathode was fabricated by blending the active materials (90 wt %), carbon black (Super P)

(4 wt %), and polyvinylidene fluoride (PVDF) (6 wt %) in n-methyl-2-pyrrolidone (NMP). The

resulting slurry was pasted onto aluminum foil and dried at 110 °C for 12 h in a vacuum oven.

The electrochemical properties were evaluated using a CR2032 coin-type half cell. The

electrolyte was 1.0 M NaClO4 dissolved in a solution of ethylene carbonate (EC) and dimethyl

carbonate (DMC) (1:2 v/v). The cells were galvanostatically charged and discharged between

2.0 and 4.0 V vs. Na+/Na at room temperature using a program controlled battery test system

(WBCS 3000S WonATech). The electrochemical impedance spectroscopy (EIS)

measurements were carried out in the frequency range of 1000 kHz to 1 mHz with an AC

amplitude of 10 mV using a ZIVE SP1 potentiostat/galvanostat/EIS.

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Fig. S1 SEM images of (Ni0.6Co0.2Mn0.2)(OH)2 precursor.

Table S1 (a) ICP-AES results of hydroxide precursor and O3- and P2-type Na-NCMs and (b)

lattice parameters of O3- and P2-type Na-NCMs determined by Rietveld refinement.

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Fig. S2 X-ray diffraction patterns of Na-NCMs synthesized at different annealing

temperature, Na content, and cooling rate.

Fig. S3 Temeprature profiles of furnace cooling and air quenching.

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Fig. S4 X-ray diffraction pattern of as-syntheized P2-type Na-NCM after re-annealing at 780 oC and furmace-cooling.

Fig. S5 Cyclic voltammogram of (a) O3- and (b) P2-type Na-NCMs at a scan rate of 0.04 mV

s-1.

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Fig. S6 Voltage profile of P2-type Na-NCM electrode between 2.0-4.3 V (vs. Na/Na+).

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Fig. S7 Average working potential vs. gravimetric specific capacity for reported P2-type

sodium layered cathode materials.

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Fig. S8 Ex-situ XANES and EXAFS spectra of pristine, 1st charged, and 1st discharged P2-type

Na-NCM electrode.

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Fig. S9. Cycling performance of the P2-type Na-NCM electrode between 2.0 and 4.2 V (vs.

Na/Na+) at the current density of 1 C.

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Fig. S10 (a, c) Nyquist plots and (b, d) relationships between real impedance and frequency for

pristince and cycled O3- and P2-type Na-NCM electrodes, respectively.

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Table S2. Impedance parameters for O3- and P2-type Na-NCM electrodes.

The inclined line at low frequency in the Nyquist plots is related to the solid state diffusion of

Na+ ions in the layered structure. The Na+ ion diffusion coefficient can be calculated based on

the following equation [1,2]

DNa+ =

𝑅2𝑇2

2𝑛4𝐹4𝐶2𝐴2𝜎2

Where R is the gas constant, T is the absolute temperature, A is the surface area of the cathode,

n is the number of electrons per molecule during oxidization, F is the Faraday constant, C is

the concentration of sodium ion, and σ is the Warburg factor which can be calulated according

to the following equation.

Zreal = RSEI + RCT + σω-1/2

Where RSEI is the resistance of the electrolyte and electrode material, RCT is the charge transfer

resistance, and ω is the angular frequency in the low frequency region. The Na+ diffusion

coefficients of P2-type and O3-type Na-NCMs obtained in this study were within the reported

range of NaxTMO2 layered oxides [3].

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Guo, Sci. Adv., 2018, 4, eaar6018.

Fig. S11 Illustration of crystal structure for O3- and P2-type Na-NCMs.